The present invention relates to a brake unit, which has at least two brake shoes, each with a friction lining, and at least one brake disc rotor, the outer surfaces of the brake disc rotor each having at least partially a friction surface composed of a metal/ceramic composite material (CMC) for respective friction linings, and at least one application device, which acts upon the brake shoes during the braking operation.
Conventional brake units, especially in motor vehicles, generally have brake disc rotors made from a cast-iron material or grey cast iron. However, the trend is towards using brake disc rotors made from a ceramic/metal composite material or for at least the friction surfaces of the brake disc rotor to be composed of a ceramic/metal composite material. Components of this kind are disclosed by DE 44 38 456 A1, for example.
When brake disc rotors of this kind are used in brake units of conventional design, however, the temperatures that occur at the friction surfaces, especially during braking operations that involve a high braking power, are significantly above those at comparable brake disc rotors made of cast iron material and cannot be tolerated by the friction linings of the brake shoes that are normally used. This results in “fading phenomena” and high wear on the brake linings.
It is therefore the object of the invention to provide a brake unit of the abovementioned type in which brake disc rotors with friction surfaces made from a ceramic/metal composite material and customary brake linings are combined in a compatible way.
The solution comprises the friction linings of the brake shoes covering at least 15% of the friction surface of the brake disc rotor, the at least one application device being designed in such a way that the pressure acting on the brake shoes acts essentially uniformly on the friction surface during the braking operation.
Thus, according to the invention, the disadvantage of the relatively poor thermal tolerance of the friction linings of the brake shoes is compensated for by increasing the surface area of the friction linings that acts on the friction surface of the brake disc rotor, the friction linings simultaneously being pressed against the friction surface of the brake disc rotor as uniformly as possible thanks to homogeneous introduction of the application forces, with the result that there is no local increase in the thermal flux density.
Advantageous developments will become apparent from the subclaims. An advantageous embodiment of the present invention consists in that the ratio of the mean height to the mean width of each friction lining of a brake shoe is approximately 1:1 to 1:1.6.
Another advantageous development envisages that an application device is provided, which acts on at least two brake shoes, at least two pistons being provided per brake shoe. The brake shoes are thus pressed into contact in such a way under the action of two or more, preferably two to four, pistons that uniform pressure is ensured, even under the action of the braking torque. In another embodiment of the present invention an application device is provided, which acts on at least four brake shoes, at least two pistons being provided per brake shoe. These are therefore application devices of comparatively simple configuration with multiple-piston callipers, preferably two-, three- or four-piston callipers. The more pistons are provided for each calliper, the more advantageous it is to provide either compressible friction linings or a compressible intermediate layer between the friction lining and the calliper, in each case preferably with a compressibility of more than 1 μm/bar brake fluid pressure.
Another advantageous refinement of the present invention consists in that a plurality of individual friction linings, each with associated individual application devices, is provided. This can be accomplished by means of single-piston callipers or multiple-piston callipers, in which one or more, preferably two to six, particularly preferably four or six, brake shoes are arranged. The friction surfaces of these brake shoes are advantageously large in the radial direction but comparatively small in the circumferential direction.
The at least one application device can furthermore have mechanical and/or electronic compensation elements, which are designed in such a way that the application forces are distributed uniformly to a plurality of friction linings using the principle of balanced levers. An improvement in the pressure is achieved by this means. An improvement in the pressure between the brake disc rotor and the brake shoes can also be achieved by using friction linings with a compressibility of over 1 μm/bar brake fluid pressure and/or an intermediate layer, provided between the friction linings and the application device, with a compressibility of over 1 μm/bar brake fluid pressure.
Especially when using more than two hydraulically actuated pistons, e.g. eight pistons, in conjunction with four friction linings per application device, it is advantageous if the pistons are arranged in such a way that the pressure acting on the brake shoes is as uniform as possible, in particular for operating friction coefficients of about 0.40 to 0.45.
To suppress braking noise, it is furthermore advantageous to configure the way in which the brake disc rotor is acted upon by the friction linings in such a way that both vibration nodes and vibration antinodes of the critical K0/3 vibration of the brake disc rotor are prevented. This is achieved by virtue of the fact that two brake shoes per friction surface of the brake disc rotor are arranged in such a way that their lines of action enclose an angle a of about 110 to 130°.
Another advantageous development envisages that at least the friction surface of the brake disc rotor or the entire brake disc rotor or the entire brake disc should be composed of a ceramic/metal composite material, preferably an aluminium/ceramic composite material, e.g. one based on aluminium oxide, titanium dioxide, boron trioxide and/or titanium boride with aluminium, as described, for example, in German patent application 197 06 925.8-45, or a silicon/ceramic composite material, e.g. one based on silicon carbide. A fibre-reinforced composite material that has carbon fibres and/or silicon carbide fibres, for example, as reinforcing fibres is particularly preferred. However, other fibres based on carbon, nitrogen, silicon or boron are also suitable.
Long fibres, preferably in the form of woven fibre structures or nonwoven scrims, are suitable as reinforcing fibres. Short fibres, preferably isotropically oriented short fibres (cf. DE 197 11 829 C1), are particularly preferred, ensuring that the friction surface and/or brake disc has isotropic, i.e. uniform, properties both in the longitudinal and in the transverse direction.
As the ceramic component, the composite material can contain a silicon carbide ceramic or an aluminium oxide ceramic, for example. However, other ceramics are also suitable.
The friction surface of the brake disc rotor and the brake disc rotor are preferably formed in one piece and are composed of the same material, i.e. of a CMC material. It is particularly preferred to produce the entire brake disc in one piece of a CMC material, making manufacture particularly simple and economical.